1. Field of the Invention
The invention relates generally to a method and apparatus for the anaerobic treatment of waste water, or flowable waste, wherein a plug flow bioreactor is utilized to treat the waste water.
2. Description of the Related Art
Many types of anaerobic microbial treatment processes have been utilized for the anaerobic treatment of soluble and particulate colloidal organic material in agricultural, domestic, and industrial waste water, or flowable waste, wherein the organic material is converted to methane, carbon dioxide, water and cell biomass, or sludge. One type of anaerobic treatment systems is the anaerobic attached film expanded bed waste water treatment system. The anaerobic attached film expanded bed technology utilizes a hydraulically expanded bed, typically a heterogeneous mixture of microorganisms, for example bacteria, which become attached to and entrapped within a bed of small diameter, biologically inert, support particles. These anaerobic attached film expanded beds are used in connection with a treatment vessel, or bioreactor.
Bioreactors, in general, may be operated in either a plug flow mode, or as a completely mixed flow bioreactor. In a bioreactor operating in a plug flow mode, the first liquid, or waste water, to enter the bioreactor is the first liquid, or treated waste water, out of the reactor. There is no longitudinal mixing of the waste water, and each unit of liquid, or waste water treated in the bioreactor remains in the bioreactor for an exact predetermined period of time, or in other words, the “hydraulic retention” time of each unit of liquid to be treated is the same. In completely mixed bioreactors, or completely mixed flow bioreactors, only the average hydraulic retention time is the same as the plug flow retention time, since some units of the liquid to be treated, or waste water, leave the bioreactor very quickly, while other units of liquid remain for a longer period of time, or for a longer hydraulic retention time.
Although it is known in the waste water treatment field that a bioreactor operating in a plug flow mode is more efficient in the removal of pollutants than a single, completely mixed bioreactor of comparable volume, the vast majority of anaerobic bioreactors have not completely achieved the desired plug flow mode, wherein the first liquid charged into the bioreactor is the first to leave the bioreactor, and “short circuiting”, or liquid particles having a shorter residence time in the bioreactor, is prevented. Undesired, excess short circuiting may occur particularly in the space above the bed of support particles through which the waste water to be treated passes upwardly therethrough. It would be desirable if an anaerobic bioreactor could be provided which better provides plug flow of the waste water throughout the bioreactor.
The fermentation of organic material to methane, carbon dioxide, and water vapor involves a consortium of anaerobic and facultative microorganisms that must be maintained in a substantially precise balance to achieve high rate, highly efficient conversion of the waste material. This consortium includes a first group of organisms, or hydrolyzers, to convert complex organic material to polysaccharides and monosaccharides. Another group of organisms are acidifiers to convert the monosaccharides to volatile acids. Lastly, the consortium includes a third group of organisms, or methanogenes, which convert the volatile acids to methane in carbon dioxide. As a given organic load is applied to a bioreactor containing all the microorganisms of the consortium, the population of each group of the consortium adjusts to produce a specific balance with the other groups. In other words, with respect to a given organic load, there would be a certain, or particular, number of hydrolyzers, a certain number of the acidifiers, and a particular number methanogenes. As the organic loading is increased, the number of microorganisms of each group typically changes, or increases, to maintain the optimal balance. The optimization of the microorganism populations requires that all the microorganisms co-exist and adjust as a group to changes in the organic loading. In general, when prior art bioreactors are operated in series, each individual bioreactor contains a separate series of individual compartments, each containing the unique consortium of the microorganisms. These bioreactors generally do not have the capability to adjust to changing organic loading in an optimal fashion. The microorganisms required for hydrolyzing are physically segregated from those responsible for acidification, and in turn those microorganisms responsible for acidification are physically segregated from those responsible for methane generation. It is believed that an optimum balance cannot be established with changing organic loading upon the bioreactor. It would be desirable if an anaerobic bioreactor could be provided with the balance of the microorganism population being readily maintained as the organic loading rate changes.